The present invention relates to a piston for a hydraulic motor having radial pistons, said piston comprising a body having a guiding and sealing surface, a base, and a top, which top is provided with a cradle-shaped recess whose edges that are situated at the top of the body offer stop surfaces for retaining a cradle-shaped journal-bearing lining that is disposed against the surface of said recess.
The cradle-shaped recess in the top of the piston serves to receive a roller or wheel designed to roll on the cam of the radial-piston hydraulic motor. While a radial-piston motor is operating, the cylinder block and the cam of said motor move in rotation relative to each other, and the pistons move radially in reciprocating motion inside the cylinders of the cylinder block, their above-mentioned rollers rolling on the cam.
For each piston, the cradle-shaped journal-bearing lining that is disposed against the surface of the recess, i.e. against the bottom thereof, receives, against it, the cylindrical surface of the roller disposed in the recess, and it is made of one or more friction-limiting materials so as to facilitate rolling of the roller.
FR 2 648 512 discloses a machine having radial pistons of the above-mentioned type. In each piston, the stop surfaces for stopping the lining are formed by shoulders that extend perpendicularly to the axis of the piston and that are obtained by machining (conventionally, by broaching) the surface of the recess. The roller is retained in the recess so that it is prevented from coming out of the recess in a direction parallel to the axis of the piston by the fact that the cradle-shaped recess extends over more than 180°, so that the edges of said recess define between them a distance that is less than the largest diametrical distance of the roller.
Such known cradle-shaped recesses, whose stop surfaces for retaining the lining are formed by machined shoulders are relatively complex to manufacture. In order to manufacture such a recess, it is necessary firstly to form the bottom of a cradle-shaped recess with a surface having the shape of a fraction of a cylinder, then to perform broaching by moving a broaching tool along the axis of said cylinder, perpendicularly to the axis of the piston, so as to cut deeper into the surface of the recess, thereby forming, in the vicinities of the edges thereof, the shoulders that face towards the bottom of the recess and that are to serve as stop surfaces. The broaching step is, in itself, costly, tedious, and lengthy.
An object of the invention is to improve the state of the art by proposing a piston that can be manufactured simply and at a lower cost.
This object is achieved by the fact that the stop surfaces for retaining the lining are formed over at least portions of the edges of the recess, in which portions the wall of the body is thinner and which portions are deformed towards the inside of the recess, by the fact that the edges of the recess also offer retaining surfaces for retaining a roller inserted into the recess, and by the fact that said retaining surfaces are formed over at least portions of the edges of the recess that are deformed towards the inside of the recess.
Thus, in accordance with the invention, the stop surfaces for retaining the lining are obtained merely by deforming edge portions of the recess inwards. Such deformations are simple and quick to implement. In addition, they can be localized on fractions only of the length of the recess, as measured along the axis of the partially cylindrical surface thereof. This makes it possible to simplify forming the deformations and to define the inside surface of the recess, as equipped with the lining, in a manner such as to avoid as much as possible having zones in relief that could be detrimental to proper rolling of the roller or wheel, or that could wear the surface thereof prematurely.
In addition, the retaining surfaces are also formed in a manner that is particularly simple.
Advantageously, a lining stop surface and a roller retaining surface are situated on the same deformation.
In this manner, the stop surface and the retaining surface are formed at the same time, at an extremely low cost.
Advantageously, at least one edge of the recess has at least one local zone in relief, namely at least one internal projection and/or at least one internal setback, extending over a fraction of the length of said edge, and advantageously situated in a middle region of said edge.
Said zone in relief can be a projection that serves as a stop surface for retaining the lining and/or as a retaining surface for retaining the roller, or else it can be a setback on either side of which two projecting surfaces serve as stop surfaces for retaining the lining and/or as retaining surfaces for retaining the roller.
The invention also relates to a method of manufacturing a piston for a hydraulic motor having radial pistons, said piston comprising a body having a guiding and sealing surface, a base, and a top, in which method a cradle-shaped recess is provided in the top of the body, and, on the edges of said recess that are situated at the top of the body, stop surfaces are formed for retaining a cradle-shaped journal-bearing lining that is disposed against the surface of said recess.
As indicated above, after the recess has been formed, known methods of obtaining such a piston require said recess to be cut deeper by means of a lengthy, tedious, and costly broaching step.
An object of the invention is to propose a method that makes it possible to avoid such a step.
This object is achieved by the fact the stop surfaces are formed by deforming towards the inside of the recess at least portions of the edges of the recess, in which portions the wall of the body is thinner, and by the fact that, also on the edges of the recess, retaining surfaces are formed for retaining a roller inserted into the recess, by deforming portions of the edges of the recess towards the inside thereof.
Advantageously, before portions of the edges of the recess are deformed, the top of the body of the piston is cut away in the vicinities of the edges of the recess so as to obtain edge zones of reduced thickness.